<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        button{
            border-radius: 10px;
        }
        div{
            width: 300px;
            height: 300px;
            margin:  0 auto;
            border: 1px solid black;
            border-radius: 10px;
        }
    </style>
</head>
<body>
    <button>按钮</button>
    <div></div>
</body>
</html>